So what is a given for the next upgrade?
1) 8th gen Intel quad core+six core
2) BT 5.0
3) RAM will remain the same up to 64 DDR4
4) Faster SSD up to 4T
5) ??
I agree with the other commenters who have replied to your post, nothing is a given in today's world.
Here is what I think will be in the next iteration of the iMac -
1) A mix of 8th Gen and 9th Gen CPUs w/ 8th Gen on the 21.5" and 9th Gen on the 27" iMac. My reasoning is that the 9th Gen CPUs may not be readily available when they are announced and since the 27" is the flagship, it will get 9th Gen first and foremost.
2) 802.11ac/BT 5.0 seems like a logical progression because the MacBook Pro series just received them.
3) I see no evidence that Intel is giving the 9th Gen a higher DRAM ceiling, but it would be a pleasant surprise. So, yes, up to 64GB of 2400MHz DDR4 DRAM.
4) Seeing as this is still Apple's main consumer desktop, I do not see them going past a 2TB SSD, but it is certainly a wildcard.
5) Radeon 500x series GPUs
6) Gigabit Ethernet, two Thunderbolt 3 ports, 4 USB 3 ports, SDXC, headphone will most likely stay the same.
Up for debate
1) User accessible DRAM slots - Because I think Apple will take the thermal lessons learned with the iMac Pro and move them down the chain, which means no more DRAM door. My guess: 50/50.
2) T2 chip - The T2 was introduced in the iMac Pro and then the MacBook Pro just recently in July. If Apple decides to incorporate the T2 chip into the iMac, it will probably be just the 27", which means it will most likely be available with only Flash Storage options. My guess: Yeah, I do not have one.
3) New enclosure - I think Apple is going to hold off on this until they get a bit more mileage out of the iMac Pro. Still silver. My guess: 0/100.
4) Display upgrades - If you consider True Tone an upgrade, sure. I think we might see HDR support come to the iMac, but what will that mean? Will Apple have a display with a larger than P3 color gamut? Will the display support HDR10, Dolby Vision, HLG? One of them, two of them, all three? Will the display be a true 10-bit? 10-bit FRC? What about the iMac Pro? Will it get the same upgrade? Too many open questions here. Frankly, I have no idea. To me, this is the real wild card.
5) If the 27" gets 9th Gen CPUs, will the Core i9-9900K (8c/16t) overlap with the entry level Xeon W-2140B too much? Again, I think Apple is going to drop the 8-core option on the iMac Pro, bring the 10-core Xenon W-2150B down to the $4999 price point and make the as yet unreleased 22-core Xeon W the top CPU for the iMac Pro.
6) USB-C ports replacing traditional USB Type-A. The Z390 PCH makes this a real possibility. My guess 40/100.
Too close to the iMac Pro?
No, and for a few reasons -
1) The iMac Pro can be upgraded to 128GB of ECC DRAM 2666MHz. The current iMac is restricted to 64GB of non-ECC DRAM 2400MHz. This is meaningful to those who need ECC and a higher ceiling than 64GB.
2) AMD Radeon Vega 56 and Vega 64 GPUs. The current iMac uses Radeon Pro 500-series GPUs and should only see a minor update to the 500X-series as the 2018 MacBook Pros did. This is meaningful for a lot of Pros who benefit from the Vega 56 and 64.
3) CPUs - The Intel Xeon W-series has a higher TDP (140w vs 95w/65w for the Core series) and more cores period.
4) Four Thunderbolt 3 ports - Only really possible on the Xeon W-series as the CPUs have x48 PCIe lanes, versus the Core CPUs that have a max of x16 PCIe lanes, which basically restricts the CPU to communicating with just the GPU as Apple does not mess with that connection. There should be enough PCIe lanes on the 300-series PCH to have two Thunderbolt 3 controllers, but Apple probably is not eager to have two TB3 controllers @x4, the Flash Storage @x4 and all of the other various items (Wi-Fi, USB 3 ports, SDXC, et al) going over the 8GT/s DMI bus, but I digress.
Although, I think the i9-9900K is going to gives the Xeon W-2140B a run for its money from a synthetic benchmark perspective, will it have the legs to keep up, or is the overlap enough that Apple slots the 10-core down to the entry level and puts 22-cores at the top end of the iMac Pro. I guess we will find out.